The invention relates to a method and apparatus for increasing the density of air being used in a compression process such as air compressors, gas turbines and other processes where the increase of air density is important. This invention also relates to a method and apparatus for reducing the temperature of air. It relates to air-conditioning. It relates to heat exchangers.
Indirect evaporative cooling invented in the mid-seventies (see, for example, Schlom, et al., U.S. Pat. No. 4,023,949) uses the latent heat of the evaporation of water to remove heat from the air. Direct evaporative coolers on the other hand, also known as swamp coolers, cool the air by passing the air through wet pads by which the evaporative absorption of the latent heat of evaporation by the water directly cools the air. Since the water vapor evaporates directly into the air, the air ends up with a heavy load of moisture. With a third approach, direct refrigeration, the air passes through a heat exchanger where it evaporates a refrigerant such as freon. The latent heat of evaporation of the freon acts on the air through a heat exchanger to cool the air. To recycle the freon additional energy is used to compress and condense the freon vapor back to a liquid, which can then be evaporated all over again. Indirect evaporation evaporates water instead of freon in the heat exchanger. Another difference is that the water vapor is not recaptured and condensed back to the liquid state, as is freon, for example, in the refrigeration case. Therefore the energy use efficiency is increased in the indirect evaporative process because no energy is expended on the condensation of the refrigerant part of a refrigeration process.
Traditionally, gas turbine intake systems depend upon the density of air to increase efficiency. Three methods, historically, to increase gas turbine intake efficiency include steam injection, refrigeration, and direct evaporative cooling. A less expensive approach to increasing the intake air density by cooling, other than these historical methods, is an indirect evaporative approach. Prior patents in this area include Schlom et al.: U.S. Pat. Nos. 4,023,949; 4,107,940; 4,137,058; 4,156,351 and 4,418,527; Fogelman: 5076347; and Kopko: WO9851916A1.
Areas in which additional increased efficiency of the indirect evaporative cooling, or the xe2x80x9cEverest Cyclexe2x80x9d (U.S. Pat. No. 4,0233,949), might be improved include (a) better heat exchange process, (b) better water evaporation process, and (c) bringing the xe2x80x9croom inlet dry-bulbxe2x80x9d as close as possible to the xe2x80x9cexhaust air wet-bulb temperaturexe2x80x9d so as to increase the thermodynamic efficiency of the actual process.
The present invention comprises novel heat exchangers and methods of constructing the heat exchangers for use in indirect evaporative cooling applications. The heat exchanger is useful for both single and multiple unit (where a plurality of heat exchangers are back-to-back) indirect evaporative processes. The evaporative apparatus for cooling comprises both a multi-stage indirect evaporative cooling heat exchanger; and a multi-stage sump where each sump stage, in a one-to-one relationship with a stage of the multi-stage heat exchanger has sump water at progressively cooler temperatures as one progresses further into the heat exchanger. Because there are separate stages of the heat exchanger and the water sumps, progressive cooling is induced on dry side output air. Other multistage heat exchangers with their associated multistage sumps can be combined, with the cooled air of a first multistage evaporative assembly feeding into the intake end of a second multistage evaporative assembly, and so on.
As a multi-stage process, for n stages, as n approaches infinity, the thermodynamic efficiency may approach a maximum, since the entropy production dS=xcexa3dq/T may be minimized (S=entropy, dq=heat transferred at temperature T). This is the case where the operational cycle of the multi-stage process is as close to a reversible process as possible. In actual practice most of the thermodynamic efficiency gain may be achieved with four to six stages.
These heat exchangers can be used for comfort cooling, pre-cooling of intake air for gas turbines and for supplying cooling air for other applications such as electrical generators and lubricating oil coolers. As a comfort, or other cooler, the present invention may be used as either a stand-alone cooling unit or as a precooler for another air conditioning device where it precools the ambient air.
As an application, the indirect-direct evaporative apparatus can be considered as an entire assembly for the pre-cooling and cleaning of the air going to a gas turbine or any other air-breathing device. As such, an additional stage is included, viz., an air washer stage in which air is directly evaporatively cooled and where simultaneously the air is also cleaned of dust, dirt and other impurities.